Apply AC to the field. With the armature stationary, adjust the brush gear for null voltage on the brushes.
While you're at it, after you have a null point, rotate the armature. A fluctuating voltage on the brushes will indicate shorted or open windings in the armature.
respectfully
The null voltage test is used for the initial brush setting. Yes electricpete, the null point will shift with the load as the magnetic field is distorted. On some motors the inter-poles do a fairly good job of maintaining the null point. Once this test is used to set the initial position of the brushes the motor is run under load. If there is sparking at one edge of the brushes, the brushes are shifted to cover the sparks.
respectfully
For a 120 volt DC field, I would use 120 V, 60HZ (or 50 Hz if that's what you have). The induction of the coil will limit the current to a much lower level than with DC excitation.
Any voltage close to the rated DC voltage.
Is this your practice also, electricpete?
respectfully
Field voltage is usually in the 100 - 300 V range and insulation is good for at least a few thousand volts. Mains is either 50 or 60 Hz.
Since the field winding is highly inductive, you can apply any normal 50 or 60 Hz voltage to any normal field winding without getting any overcurrent, and the insulation can take any normal voltage.
The normal wall outlet voltage is the easiest way of getting an AC voltage. But remember that you are dealing with lethal voltages. All safety precautions apply.
